If the items are in your inventory, you want to sell when it's red. Red indicates higher than average price, yellow average, green lower.
Buy green, sell red.
Buy sumpter horses off villagers coming back from market, don't pay more than 25D for each. Once you have a herd, you can carry 5k+ of inventory. Start off with cheap stuff, get variety, and wander the map. Selling to caravans and villages tends to work out better, as they're prices don't vary when you sell to them.